This invention relates generally to plasma diagnostics and process control in depositing thin films on substrates, particularly those processes utilizing sputtering, plasma enhanced chemical vapor deposition (PECVD) and plasma polymerization.
As is well known, a thin film depositing plasma is formed in a chamber by introducing at least one gas into a region of a controlled electrical field. Many such plasma processes operate at low pressure with magnetic confinement. Most plasma processes involve a control of at least the internal pressure level, the electrical field characteristics, and the composition and proportional flow rates of individual gases into the plasma. Selection of these variables, in turn, affects the properties of a resulting thin film. Such properties can include the film's hardness, its adhesion to the substrate, its permeability to certain liquids or gases, optical characteristics of translucence and refractive index, and its general composition. The property or properties of the resulting film that are important depend upon the purpose and application of the resulting product. For example, if a scratch resistant coating is being applied to glass, the film's hardness, adhesion to glass and degree of optical clarity are the most important properties. In another example, wherein a coating is desired to prevent the permeation of oxygen, that property of the thin film is most important.
It is, of course, desired to control the plasma variables in order to produce a product with the desired film properties. Heretofore, most process control has been manual, based upon some but incomplete measurement of the resulting plasma characteristics. One such technique is to measure the electron temperature (T.sub.e) of the plasma, which is a measure of the average electron energy in the plasma, by the use of available Langmuir (electrostatic) probe(s) positioned in the plasma. The plasma variables are then manually adjusted until the average electron temperature corresponds to that which has been determined to be necessary for obtaining the desired film properties or rate of deposition of the film on the substrate. However, since the Langmuir probe(s) must be positioned in the plasma, they quickly become coated with the film being deposited and its readings then are subject to considerable error. Also, such an average electron temperature measurement provides only a partial picture of the plasma's characteristics which, in some thin film processes, is inadequate.
It is still the practice in large-scale commercial thin film deposition processes to adjust the plasma variables to a combination that is believed by the operator to be optimum for a particular application, and then to run and test a sample. Only when the plasma variables have been readjusted in response to many such test cycles is the plasma process adjusted for commercial runs.
Therefore, it is a primary object of the present invention to provide improved monitoring and control of the plasma process in order to provide a higher yield of coated product having films with uniform and repeatable properties.
It is another object of the present invention to provide a plasma thin film deposition process that is suitable for continuous commercial use in the coating of large substrates such as automobile and architectural glass.